Structural hierarchies at the molecular level can be determined by the hierarchies of interaction energies, in other words, for the generation of complex structures in several steps significant differences in the interaction energetics are required. Whereas the assembly of the first structural level needs to involve strong interactions between the molecular building blocks, its subsequent extension without modification of the scaffold is conveniently achieved on the basis of weaker bonding forces. This applies both to supramolecular assemblies in solution and in air/vacuum and to fixed or spacially confined structures.
We recently reported the formation of a hexagonal molecular network generated by thermal dehydrogenation of 4,9-diaminoperylene-quinone-3,10-diimine (DPDI) on a Cu(111) surface [1]. By thermal activation, these molecules form autocomplementary hydrogen-bond donors/acceptors which preposition themselves in the formation of the surface network. The highly regular honeycomb structure is commensurate with the Cu substrate [in the form of a p(10×10) superlattice with a lattice constant of 2.55 nm] and thermally highly stable (up to > 300°C) as a consequence of a combination of strong pi-bonding between the organic molecules and the surface metal atoms and resonance-assisted H-bonding between the molecules. Due to its structural regularity and stability, this surface structure provides the ideal starting point for the assembly of functional hierarchic aggregates.
On the one hand, octaethylporphyrin (OEP) molecules were incorporated in the "holes" of the hexagonal network. OEP displays a thermally activated hindered rotation whereas the reorganization between the potential minima, a rotation-libration, is monitored in the STM tunnelling current as a bi-state "switching" [2]. On the other hand, for C60 trapped in the hexagonal pores it was of interest to find out if and how they can be moved from one pore to another. By using the tip of an STM operated under UHV and at low temperature it turned out that both vertical and horizontal manipulation can be applied to re-position individual C60.
[1] M. Stöhr et al., Angew. Chem. Int. Ed., 2005, 44, 7394
[2] M. Wahl et al., accepted in Chem.Commun. |